Biomass pellet fuel heating device, system and method

ABSTRACT

There is provided a pelletized-biomass combustion device comprising: (a) an elongate upright combustion capsule; (b) a grate element disposed within and bisecting the combustion capsule, (i) the grate element having a first side and a second side and (ii) the grate element open to fluid communication between the first side and the second side; (c) a first combustion air inlet facing the first side of the grate; (d) a second combustion air inlet facing the second side of the grate; (e) a fuel-feed inlet; and (f) a flue-gas exhaust outlet. The combustion capsule can be formed of two mated components such as a chamber housing mated with a burn pot member, or instead it can be formed as a single unit such as the chamber housing bisected by the grate. In any event, the grate element has two sides, and there are two combustion air inlets, each facing opposite sides of the grate. The combustion device is preferably an induced-draft system wherein the exhaust fan is positioned at the distal end of the stove pipe.

BACKGROUND OF THE INVENTION

The present invention relates to biomass pellet fuel heating devices, some of which are at times called pellet burners, pellet burning stoves or pellet stoves, and related systems and methods.

Pellet fuel heating devices are used in the USA and other countries. These heating devices, which include without limitation stoves, boilers and furnaces, burn pelletized wood and/or biomass fuels to produce heat. In comparison to natural-form wood burning devices, pellet burners generally have lower emission levels and higher combustion efficiencies. Pelletized fuel is also cleaner and easier to handle than natural-form wood, and is often derived from agricultural and industrial waste materials, including the waste of the timber industry.

The efficiencies and environmentally-friendly features of current pellet fuel heating devices drive their rising popularity, but drawbacks and inefficiencies remain.

The combustion of pellet fuels still produces wood ash and particulates. Particulate matter is a primary component of smog and atmospheric pollution, and therefore its generation and release into the atmosphere is an environmental negative. The combustion of pellet fuels also generates secondary sources of particulate pollution, such as sulfur dioxide, nitrogen oxides, ammonia and VOCs (volatile organic compounds). A further reduction of these pollution sources requires greater combustion efficiencies than has been seen to date in pellet fuel heating devices. Increasing the combustion efficiency of a heating device reduces emissions of primary and secondary sources of particulate pollution. In other words, pellet fuel heating devices with increased combustion efficiences are cleaner. They are not only environmentally cleaner, they produce less wood ash and therefore are themselves cleaner to operate and maintain.

General consumer acceptance of pellet fuel burning devices depends in significant part on whether or not the devices present any safety issues. Current designs for pellet stoves, for instance, call for the storage of the pelletized fuel in hoppers positioned behind the combustion chamber or to the side of the stove. Many pellet stoves use reverse-angle screw auger delivery systems in which the pellets are transported from the bottom of the hopper at about an upward 45 degree angle to a chute positioned at about a 45 degree reverse angle. The pellets are released from the auger screw at the top of the chute, and slide down the chute into the device's combustion chamber. Other delivery systems in current use employ hollow tubes or feed plates which move the pellets horizontally from the bottom of hoppers directly into the combustion chambers.

All of the current pellet delivery methods present a significant burnback risk. Burnback is a serious and dangerous phenomenon in which the pellets within the hopper are ignited. Burnback can occur when there is a sufficient trail of fuel between the combustion chamber and the hopper to bridge the two areas. In other words, burnback can occur whenever there is a sufficient fuel continuum between (a) the fire in the combustion chamber and (b) the fuel in the hopper. Such a fuel continuum can occur when the delivery pathway between the hopper and the combustion chamber is clogged with fuel, or fuel build-up. Burnback is possible with current pellet fuel delivery systems because they allow an unbroken line of fuel pellets to flow from the hopper directly into the combustion chamber.

Some current pellet fuel heating devices use just a natural draft system for exhausting flue gases, normally with poor results. Natural draft systems are often used in common wood stoves. In natural draft systems, the heat of the fire forces the air in the stovepipe to exhaust into the exterior environment. A stovepipe's diameter is proportionate to the size of the combustion chamber and/or the size of the fire customarily therein. Stovepipe diameters of most common wood stoves range from about six to about eight inches. In contrast, the combustion chambers of current pellet stoves and the fires therein, are smaller than wood stoves, and the stovepipe diameters are generally only about three to four inches, which are about half the diameter of wood stoves. Ash and particulate build-up inside such small-diameter stovepipes quickly leads to draft reduction, which in turn brings about poor combustion and reduced thermal output.

To improve the draft, thereby improving the air to fuel ratio in the burn or combustion chamber and reducing particulate build-up inside stovepipes, current pellet fuel heating devices often use forced-draft blower systems as enhancements to natural draft systems. Forced-draft blower systems are traditionally located within the pellet stoves themselves. These blower systems either force air into the burn chamber, or forcibly draw air out of the burn chamber. In either instance, the blower systems forcibly push the flue gases from behind into and through the stovepipe, and then out to the building's exterior.

Current forced-draft blower systems increase the air transfer, augmenting the combustion process in the burn chamber, but have significant drawbacks. One drawback is the sound of the exhaust fans of these draft blower systems. They are typically sufficiently noisy to be an irritant in residential and workplace settings.

Another drawback of current forced-draft blowers is the risk of contamination of the interior environment presented by pushing flue gases into a stovepipe. A stovepipe typically runs through the room, from the heating device to an exit point, which is the place or spot at which it penetrates the ceiling or one of the walls and from there exits to the outside. A significant length of stovepipe therefore is exposed within the room. Pushing air into the stovepipe forces flue gases through tiny gaps at the pipe seams and through cracks or tiny holes along the length of the pipe. Flue gases routinely contain carbon monoxide and other pollutants, such as wood ash. Even a minor escape of flue gas from a stove pipe into a building's interior environment poses a dangerous health hazard.

SUMMARY OF THE INVENTION

The present invention provides a pellet fuel heating device and system and method in which pellet fuel is combusted in a close mode, that is, in a constricted burn chamber, whereby thermal efficiency is dramatically increased while particulate emissions are substantially reduced.

The present invention also provides a pellet fuel heating device and system and method in which pellet fuel is combusted in a stoke mode, that is, in a dynamic environment, under stimulation or agitation force, whereby collection of pellets in heaps is avoided, leading to more complete combustion and reduction of particulate emissions.

The present invention in some embodiments also provides a pellet fuel heating device and system and method in which pellet fuel drops unhindered and unimpeded through a vertical feed conduit into the burn chamber, whereby the formation of a pellet continuum is precluded and the risk of burnback is eliminated. In some embodiments, a vertical pellet feed tube is coincident with an upper combustion feed tube whereby the pellet fuel is carried with a fast-flowing combustion air stream as it drops into the burn chamber.

The present invention also provides a pellet fuel heating device and system and method with an induced air draft system positioned at the stovepipe's external distal end, whereby the need for any forced-air blower within the core pellet stove is eliminated, together with the irritating sound of a fan operating within the core pellet stove and the risk of contamination of the interior environment presented by pushing flue gases into a stovepipe.

Other features and advantages of the present invention, including without limitation both an upper combustion air inlet and lower combustion air inlet, both inlets facing the grate which supports the burning pellets, are described below.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partially diagrammatic, partially exploded, perspective view of a pellet fuel heating device and system of the present invention.

FIG. 2 is a partially diagrammatic side view of a biomass pellet combustion device and system of the present invention.

FIG. 3 is a partially diagrammatic, partially exploded, perspective view of a part of a pellet stove of the present invention.

FIG. 4 is a partially diagrammatic view of a reverse-direction combustion chamber of a pellet stove of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 there is shown a pellet fuel heating device, which for simplicity is generally referred to herein as a pellet stove, designated generally by the reference numeral 10. The pellet stove 10 as shown includes a burn pot 11 and a tubular chamber housing 12. The burn pot 11 includes a flat grate 14 and a pot member 16. As shown, and in preferred embodiment, the pot member 16 is an open-topped cylindrical receptacle with a surround or rim 18 encircling an open, upwardly facing, mouth 20.

The grate 14 is shown in FIG. 1 elevated above the pot member 16 for illustration purposes. In use the grate 14 is seated in the pot member 16 as described in more detail below. The chamber housing 12 is shown in FIG. 1 spaced apart from the pot member 16 for illustration purposes only. In use the chamber housing 12 is mated with the pot member 16. The chamber housing 12 and pot member 16, in combination, form, determine and/or enclose a burn or combustion chamber 22.

As shown, and in preferred embodiment, the combustion chamber 22 is a close-mode or constricted burn chamber. The combustion chamber 22 is not a conventional hearth or spacious cabinet-like chamber used in conventional pellet stoves. As shown in FIG. 1, the combustion chamber 22 is only about four inches wide and about thirty inches long (high). The inside diameters of both the vertical tube 12 and the pot member 16 are only about four inches. As shown in FIG. 1, the diameter of the grate 14 approaches the inside diameter of the pot member 16. The close-mode combustion chamber 22 permits greater thermal efficiencies than the efficiencies possible with current pellet stoves. The close-mode combustion chamber 22 also provides a significant particulate emissions reduction over emissions given out by current pellet stoves.

The pellet stove 10 shown in FIG. 1 includes an elongate, substantially vertical or upright, air/pellet feed tube 24. The air/pellet feed tube 24 is an open-ended conduit through which pellets (not shown) substantially free fall down from an elevated pellet-feed device (not shown). The falling pellets exit the air/pellet feed tube 24 at its bottom open end 26. The open end 26 of the air/pellet feed tube 24 is sufficiently spaced above the grate 14, and the pellet feed tube 24 is sufficiently long, that the pellets land upon, and to some extent actually bounce around, the grate 14, whereby a substantially single layer of pellets takes form. A pile or other accumulation or buildup of pellets on the grate 14 is neither required nor desirable for the pellet stove 10 of the present invention and the combustion efficiencies obtainable therewith.

As shown, and in some embodiments of the invention, the open end 26 of the air/pellet feed tube 24 is spaced above the four-inch diameter grate 14 a distance of about ten inches when the pellet stove 10 is in operational assembly. In such an embodiment, a burn chamber center 23 bounded by the grate 14, the chamber housing 12, and the horizontal plane of the open end 26 of the air/pellet feed tube 24 is demarcated. The volumetric size of the burn chamber center 23 is about one hundred twenty-five cubic inches.

The pellet stove of the present invention, including without limitation the pellet stove 10 shown in FIG. 1, is not subject to any significant risk of a burnback event. As mentioned above, burnback is a serious and dangerous phenomenon in which the pellets within the hopper are ignited. Burnback can occur when there is a sufficient trail of fuel between the combustion chamber and the hopper to bridge the two areas. In other words, burnback can occur whenever there is a sufficient fuel continuum between the fire in the combustion chamber and the fuel in the hopper. Such a fuel continuum can occur when the delivery pathway between the hopper and the combustion chamber is clogged with fuel, or fuel build-up. Burnback is possible with current pellet fuel delivery systems because they allow an unbroken line of fuel pellets to flow from the hopper directly into the combustion chamber.

The pellet stove of the present invention, including without limitation the pellet stove 10 shown in FIG. 1, is not subject to any significant risk of a burnback event because there cannot be any fuel continuum between the fire in the combustion chamber and the fuel in the hopper. In other words, the delivery pathway between the hopper and the combustion chamber cannot, under normal operation, become clogged with fuel, or other fuel build-up. The pellets used with the pellet stove 10 shown in FIG. 1 fall freely down through the substantially vertical air/pellet feed tube 24. As shown in FIG. 1, the air/pellet feed tube 24 has an inside diameter of about 1.25 inches. Commercial fuel pellets are about 0.25 inches wide and from about 0.5 to about 0.75 inches long, which is quite suitable for use with air/pellet feed tube 24. Free falling pellets of such size will fall freely down the air/pellet feed tube 24. The drop from the open end 26 of the air/pellet feed tube 24 down to the grate 14 is about ten inches. The dropping pellets form a substantially single layer on the grate 14. Therefore the gap between pellets burning on the grate 14 and the end 26 of the feed tube 24 is almost ten inches. There is no train or line of pellets moving to the combustion chamber 22, and therefore there can be no continuous train or unbroken line of fuel pellets running from the hopper to the combustion chamber 22. There is no point along the delivery pathway between the hopper and the combustion chamber 22 that can become clogged with fuel, or suffer fuel build-up. The risk of burnback under normal operation of a pellet stove of the present invention is thereby reduced to negligible.

The pellet stove 10 shown in FIG. 1 includes two combustion-air inlets into the combustion chamber 22, namely one below the grate 14 and one above the grate 14. In more detail, the pellet stove 10 has a main combustion air conduit 28 leading in from the left of FIG. 1. The main combustion air conduit 28 forks into a first or top combustion air conduit 30 and a second or bottom combustion air conduit 32. The top combustion air conduit 30 opens into the air/pellet feed tube 24 at an air-feed inlet 31 downstream of the pellet feed point 27 and upstream of the open end 26 of the air/pellet feed tube 24. The air/pellet feed tube 24 in this embodiment therefore can be considered the effective distal section of the top combustion air conduit. A coextensive length of pellet feed conduit and air feed conduit, as illustrated in FIG. 1, is provided in some, but not all, preferred embodiments of the present invention, as demonstrated by other embodiments described below.

The bottom combustion air conduit 32 opens into the bottom of the pot member 16 beneath the grate 14. The bottom combustion air conduit 32 as shown and in some preferred embodiments is substantially centered on the bottom of the pot member 16. Intersecting the bottom combustion air conduit 32 at a point where it runs vertically up to the pot member 16 is an ash collection member 21 comprised of an ash collection housing 21 a and a normally-closed ash drawer 21 b (shown partly opened for cleaning) that does not impact combustion air flow. As shown in FIG. 1, and in some embodiments of the invention, the pot member 16 is about 3.5 inches high. Therefore the grate 14 of the pellet stove 10 shown in FIG. 1, when seated with the pot member 16, will be positioned between the two combustion air inlets, namely the open end 26 of the air/pellet feed tube 24 and the bottom combustion air inlet 34. Further, as shown in FIG. 1, and in preferred embodiments of the invention, the air/pellet feed tube 24 and the bottom combustion air inlet 34 not only lie along the same axis, such axis lies substantially normal to, and through, the center of the grate 14. As shown in FIG. 1, and in some embodiments, the pot member 16 is fabricated together with a bottom hollow pedestal stem 36. The pedestal stem 36 is the effective final section of the bottom combustion air conduit 32 and as shown in bisected by a jack screw 17. The bottom combustion air inlet 34 is the orifice at the point where the pedestal stem 36 opens into the pot member 16. As shown, and in some embodiments of the invention, the bottom combustion air inlet 34, and pedestal base 36, have an inner diameter of about 1.25 inches.

As shown, and in preferred embodiments, the cross section inside diameters of the top combustion air conduit 30 and the bottom combustion air conduit 32 are appreciably less than the cross-section inside diameter of the combustion chamber 22. As depicted in FIG. 1, the cross-section inside diameters of the top combustion air conduit 30 and the bottom combustion air conduit 32 are about the same, and both are from about one-fourth to about one-third of the cross-section inside diameter of the combustion chamber 22. The downwardly concave or hemispherical configuration of the bottom 38 of the pot member 16 has a significant role in preferred embodiments of the present invention. Embers that fall through the grate 14 swirl around the bottom 38 of the pot member 16 until they burn up. As mentioned above, the pot member 16 is about 3.5 inches high and about 4 inches wide at its upper mouth 20. The width of the pot member narrows from top to bottom with the hemispherical configuration of its bottom 38. The shape of the bottom 38 of the pot member 16, together with the bottom entry of combustion air through the bottom combustion air inlet 34, provides a type of whirlpool effect on dropping embers. If the bottom 38 of the pot member 16 were instead formed with vertical sides and a flat bottom, the sediment from falling embers would clog up bottom combustion air inlet 34 despite the realization of almost complete pellet combustion, as described below.

The pellet feed tube 24 as shown in FIG. 1 and in preferred embodiments is concentric with the tubular chamber housing 12 and substantially centered within the combustion chamber 22. The outer diameter of the air/pellet feed tube 24 is not significantly greater than the inside diameter. Flue gas rises in the combustion chamber 22 in the ringed space between the outer surface of the air/pellet feed tube 24 and the inner surface of the tubular chamber housing 12. At its upper end, the vertical tube 12 has a wall or restriction member 13 with an orifice 15 that is about 3 inches wide. The combustion chamber 22 is open to, and exhausts to, a heat exchanger 42 through the orifice 15 in the restriction member 13. The heat exchanger 42 is shown only diagrammatically in FIG. 1 because heat exchangers are both well known and diverse, and it is well within the skill of a person with ordinary skill in the art to select a suitable heat exchanger for use in combination with the other elements of the pellet stove 10. The flue gases pass through the heat exchanger 42 and then to an exhaust conduit or stovepipe 40 that runs to an exit point (not shown). An exhaust fan (not shown) is positioned at the external distal end of the stovepipe 40, which draws flue gases and any entrained particulates out of the system.

The air/pellet feed tube 24 as shown in FIG. 1 and in some embodiments has a horizontal cross-section area diameter of about 1.2 square inches, which takes up about one-tenth of the approximately 12.5 square inches of the cross-sectional area of the tubular chamber housing 12. This leaves about cross-sectional area of about 10 square inches or so for the passage of flue gases up and out of the combustion chamber 22 through the stovepipe 40.

The fuel pellets (not shown) are combusted on and/or slightly above the grate 14. Combustion air inflow from below the grate 14, together with the rising flow of flue gases, are routinely sufficient to lift or float pellets, particularly partially consumed pellets, slightly off and above the grate 14. Such a fire is being fed oxygen-containing combustion air from both above and below, and the flue gases emanating from the fire are discharged from the combustion chamber 22 to the heat exchanger 42 and then through the stovepipe 40. During the operation of the pellet stove 10, each individual pellet of the single-layer of fuel pellets will burn to the point of almost complete combustion. In addition, the flue gases themselves will combust as they emanate from the ignited pellets. Superior combustion, and superior thermal energy, is produced by combining gravitational force and kinetic energy in the downward motion of fresh pellets and combustion gas through the air/pellet feed tube 24. Further, the fresh pellets will strike the grate 14 or burning pellets on the grate 14 with greater kinetic energy because they are being fed entrained in the fast-flowing combustion air stream of the combined air/pellet feed tube 24. (Pellets fed through a vertical feed tube separate from an air feed tube would strike a grate with less kinetic energy.) As fresh pellets and fresh combustion air strike the burning pellets from above, they restoke the combustion bed every few seconds, thereby causing continuous flux within the combustion chamber 22. Such a stoke or dynamic mode results in complete, or near complete, burning of the pellets and their embers. Such combustion efficiencies vastly increase thermal output and dramatically reduce the fly ash and particulate content of the flue gas. The pellet stove 10 of the present invention, if operated with at least a modicum of proficiency, requires at most only a small ash collection device, such as the ash collection member 21 shown, beneath the grate 14 to contain accumulating ash and particulates as the fire burns, simply because little or no ash or particulates accumulate.

The grate 14 is open framework of bars or wires having a structural integrity that is sufficient to support the pellets on its upper side. As shown, and in preferred embodiments of the present invention, the grate 14 is level, i.e., horizontal with a substantially even flat top surface. By substantially even and flat is meant that there are substantially no hollows or other concave depressions of sufficient dimensions wherein pellets could lodge and build up atop of one another. The structural elements of any open framework, such as the grate 14, may themselves present minor ridges and/or small hollows between the elements provided that they sufficiently small and/or shallow to substantially avoid pellet build-up.

The flat horizontal design of the grate 14 has been surprisingly found to provide optimum burn characteristics. It allows the pellets to combust individually, side by side, rather than in a heap such as the pellet-atop-pellet heaps required to maintain combustion in conventional pellet stove. The side-by-side combustion of pellets of the present invention achieves substantially complete pellet combustion without formation of fused ash. Fused ash is formed from dirt, other non-combustibles and/or byproduct contaminants that are mixed with, and/or incorporated into, biomass fuel pellets during manufacturing. When fuel pellets are burned in conventional pellet stoves, such noncombustible materials fuse together in clumps that are called fused ash. In the present invention with a confined burn chamber and a dynamic combustion mode wherein the combusting fuel pellets are exposed to constant agitation, fused ash does not form. In the absence of fused-ash formation, the grate 14 does not become coated or clogged with fused ash, and the grate 14 does not require frequent and/or repetitive cleanings.

In addition, the grate 14 has sufficient gaps between its structural elements across its entire effective pellet-support surface to accommodate combustion air flow from the bottom combustion air inlet 34. In other words, combustion air preferably should permeate the grate 14 from below substantially evenly across the entire pellet-support surface of the grate 14.

Fuel pellets are typically about 0.25 inches wide and from about 0.5 to about 0.75 inches long, and weigh from about 3 to about 5 grams each. A conventional pellet stove burns about two pounds of pellets an hour at what is considered a low-burn setting, and from about five to about six pounds of pellets per hour at a high-burn setting. A conventional pellet stove cannot be operated at a burn rate lower than about one pound of pellets per hour because the fire will just go out. Pellet stoves of the present invention, such as the pellet stove 10 shown in FIG. 1, burn about 0.5 pounds of pellets an hour at a low-burn (pilot) setting, which is such a low feed rate setting that it cannot be realized in conventional pellet stoves. Such a pilot setting permits a pellet stove of the present invention to run-on-pilot, without getting totally cold, about eighty hours on a typical forty-pound bag of pellets. A pellet stove of the present invention, such as the pellet stove 10 illustrated in FIG. 1, can be fired up to heat-emitting status from the pilot setting in about five minutes. In contrast, firing up a conventional pellet stove requires re-ignition, which means there will be a delay time of from about ten to fifteen minutes before the heat-emitting stage is reached.

Further, a pellet stove of the present invention, such as the pellet stove 10 shown in FIG. 1, burns about 5 pounds of pellets per hour at a high-burn setting, while providing about the same BTU output as conventional pellet stoves burning 5 to 6 pounds (2500 to 3000 grams) of pellets per hour. The feed or burn rate of pellets to 4 inch diameter pellet stoves of the present invention, such as the pellet stove 10 shown in FIG. 1, is about 250 grams per hour on a pilot (low burn) setting, 500 grams per hour on a low setting, 1,000 grams per hour on a medium setting, and 2,000 grams per hour on a high setting, noting that 500 grams is about 1.1 pounds. Pellet stoves of the present invention having larger diameters would burn proportionally more pellets per hour at a given setting. It is noted that most conventional pellet stoves will not continue to burn pellet-feed rates of less than one pound per hour. The feed rate of the pellets for both conventional pellet stoves and for pellet stoves of the present invention is typically an automatic, substantially continuous, process that is controlled by an electronic controller or similar device. There are a number of commercial manufacturers of electronic controllers, such as Oven Industries of Mechanicsburg, Pa. (www.ovenind.com), which provides both standard temperature sensors and controllers for heating applications and designs and manufactures custom or OEM electronic controllers and sensors. It is well within the skill of a person of ordinary skill in the art to select and/or design a suitable controller for a pellet stove of the present invention.

Current commercial fuel pellet stoves at maximum reach a combustion chamber external surface temperature of about 600° F. Pellet stoves of the present invention, such as the pellet stove 10 shown in FIG. 1, provide combustion chamber external surface temperatures of about 600° F. on a low-burn pilot setting to 1,600° F. on a high-burn setting. Such external surface temperatures are not possible in conventional pellet stoves because the chamber wall is too far from the fire or burn pot. The internal temperatures of a pellet stove of the present invention, such as pellet stove 10 shown in FIG. 1, range from about 600° F. at the level of the burn grate to more than 2,000° F. through all burn levels.

The burn chamber wall or housing of a pellet stove of the present invention, such as the chamber housing 12 of the pellet stove 10 shown in FIG. 1, is preferably formed of steel or other heat resistant material that has a thickness of from about 0.06 to about 0.4 inch. More preferably the thickness of the burn chamber wall or housing should be less than about ⅜ of an inch. The burn chamber wall or housing of a pellet stove of the present invention can be thicker, for instance ¾ of an inch thick, but it is believed there will seldom be a reason for using such a thick burn chamber wall or housing. In addition, thick walls have been found to be deleterious to the pellet stove of the present invention because they only act as heat sinks without providing any concomitant advantage.

A biomass pellet combustion device such as a pellet stove consumes a great amount of oxygen. The high oxygen consumption is a primary reason pellet stoves are conventionally outfitted with combustion air blowers. The biomass pellet combustion device of the present invention, such as the pellet stove 10 illustrated in FIG. 1, works off the dual above/below combustion air feed system described above in combination with a remote exhaust fan at or about the distal end of the stovepipe (stove pipe is also called an exhaust pipe or chimney). The feed rate of pellets to such a pellet stove 10 is controlled automatically, by well known and conventional pellet stove feed controls. The selection of a suitable pellet stove feed control is well within the skill of a person of ordinary skill in the art. The exhaust fan at or about the distal end of the stovepipe in embodiments of the present invention would be set for a sufficient air movement rate. The air movement rate or CFM (cubic feet per minute) of the exhaust fan is directly proportional to the size of the burn chamber. A 70 or 80 CFM exhaust fan is sufficient for burn chambers up to about eight inches in diameter, while a larger CFM fan should be used for larger burn chambers.

Many of the features and components described above for the pellet stove 10 shown in FIG. 1 do not require repetition, and will not be repeated, for the other embodiments of the present invention described below.

Referring now to FIG. 2 there is shown a pellet fuel heating device designated generally by the reference numeral 110. The pellet stove 110 as shown includes a box-shaped burn pot 111 and an upright chamber housing 112 having a substantially square cross-sectional profile. The burn pot 111 includes a flat and substantially square grate 114 and a pot member 116. As shown, the pot member 116 is an open-topped receptacle having a substantially square cross-sectional profile. The pot member 116 has a rim 118 or surround running along all sides of the pot member 116. Neighboring and inward of the rim 118 is a continuous ledge 119 which, like the rim 118, runs along all sides of the pot member 116. The inner edge of the ledge 119 forms the perimeter an upwardly-open mouth 120 of the pot member 116.

The upright chamber housing 112 is shown in FIG. 2 in its operational placement, namely mated with the pot member 116. In the embodiment shown in FIG. 2, the chamber housing 112 is seated inside of the rim 118 on the inner ledge 119 of the pot member 116, whereby the mating of the chamber housing 112 and the pot member 116 is realized. The grate 114 as shown in FIG. 2 is also seated on, and supported by, the inner ledge 119 of the pot member 116. The grate 114 is seated on an inner section of the ledge 119 adjacent to, but not beneath, the chamber housing 112. In other words, the bottom of the chamber housing 112 neighbors the perimeter edge of the grate 114. The chamber housing 112 and pot member 116, in combination, form, determine and/or enclose a burn or combustion chamber 122.

As shown, and in preferred embodiment, the combustion chamber 122 is a close-mode or constricted burn chamber. The combustion chamber 122 is not a conventional hearth or spacious cabinet-like chamber used in conventional pellet stoves. As shown in FIG. 2, the burn chamber 122 is only about five inches square and about thirty-eight inches long (high). The inside dimensions of both the chamber housing 112 and the pot member 116 are only about five inches along each side. As shown in FIG. 2, the dimensions of the grate 114 approaches the inside dimensions of the pot member 116. The close-mode combustion chamber 122 permits greater thermal efficiencies than the efficiencies possible with current pellet stoves. The close-mode combustion chamber 122 also provides a significant particulate emissions reduction over emissions given out by current pellet stoves.

The pellet stove 110 shown in FIG. 2 includes a pellet feed tube 124. The pellet feed tube 124 is an open-ended conduit through which pellets 123 are routed from a pellet hopper 125 to a pellet inlet 144 in the chamber housing 112 at a point overhead of the grate 114. The pellets 123 are conveyed from the hopper 125 up into the pellet feed tube 124 by a conventional auger screw device or the like (not shown). The pellets 123 are pushed up through the pellet feed tube 124 until they start falling down to the grate 114.

The pellet stove 110 shown in FIG. 2 includes an elongate top combustion air conduit 130 that runs through an perimeter wall 146 over to a point above the combustion chamber 122, through the otherwise closed chamber housing 112, and then down through the combustion chamber 122, terminating at a point substantially centered above the grate 114. The portion of the top combustion air conduit that is disposed inside of the chamber housing 112 when the pellet stove 110 is in operational assembly can be considered a top combustion air feed tube 148.

The embodiment shown in FIG. 2 as described so far differs from the embodiment shown in FIG. 1 in several respects, including without limitation (1) the square rather than circular cross-sectional profiles of the pot member 116, the grate 114 and the chamber housing 112, (2) the presence of separate, rather than coextensive, top combustion air feed tube 148 and pellet feed tube 124, and (3) the running of the top combustion air conduit 130 from the perimeter wall 146, rather than its forking off of a single combustion air conduit that feeds both the top combustion air conduit and a bottom combustion air conduit (introduced below). Further, the vertical sides and the flat bottom of the pot member 116 will not provide the swirling of embers that fall through the grate 114, as described regarding the pot member 116 of the pellet stove 10, and therefore some type of sediment collection device preferably should be provided at the bottom or below the pot member 116. These variations are not believed as advantageous as the features shown in, and described with respect to, the pellet stove 10 of FIG. 1, but nonetheless are well within broad embodiments of the invention. Individually or in combination these variations may find different uses and benefits not delineated herein and still provide at least some of the advantages of the present invention and thus remain within the present invention.

The top combustion feed tube 148 has a distal bottom open end 126. The open end 126 of the top air feed tube 148 is sufficiently close to the grate 114, and the pellet feed tube 124 opens into the combustion chamber 122 at a sufficiently elevated point, that the pellets 123 land upon, and to some extent actually bounce around, the grate 14, whereby a substantially single layer of pellets under constant agitation takes form. A pile or other accumulation or buildup of pellets on the grate 114 is neither required nor desirable for the pellet stove 110 of the present invention and the combustion efficiencies obtainable therewith.

As shown, and in some embodiments of the invention, the open end 126 of the top combustion air feed tube 124 is spaced above the five-inch diameter grate 114 a distance of about twelve inches when the pellet stove 110 is in operational assembly as shown in FIG. 2. In such an embodiment, a burn chamber center 127 bounded by the grate 114, the chamber housing 112, and the horizontal plane of the open end 126 of the top combustion air feed tube 124 is demarcated. The volumetric size of the burn chamber center 127 is about three hundred cubic inches.

The pellet stove 110 shown in FIG. 2, is not subject to any significant risk of a burnback event because the pellets 123 free fall from the elevated pellet inlet 144.

The pellet stove 110 shown in FIG. 2 includes two combustion-air inlets into the combustion chamber 122, namely the inlet comprised of the open end 126 of top combustion air feed tube 148, which is above the grate 114, and one below the grate 114. In more detail, a second or bottom combustion air conduit 132 runs through the perimeter wall 146 over to a point below the burn pot 111, then up through the otherwise closed bottom of the pot member 116. The bottom combustion air conduit 132 opens into the bottom of the pot member 116 beneath the grate 114 at a bottom combustion air inlet 134. The bottom combustion air inlet 134 as shown and in some preferred embodiments is substantially centered on the bottom of the pot member 116. Below the bottom combustion air inlet 134 as shown is a jack screw 117, and below the jack screw 117 is an ash collection member 212 comprised of an ash collection housing 121 a and a normally-closed ash drawer 121 b (shown partially open for cleaning). As shown in FIG. 2, and in some embodiments of the invention, the pot member 116 is about 3 inches high. Therefore the grate 114 of the pellet stove 110 shown in FIG. 2, when seated on the ledge 119 in the pot member 116, will be positioned between the two combustion air inlets, namely the top combustion air inlet at the open end 126 of the top combustion air feed tube 124 and the bottom combustion air inlet 134 at the end of the bottom combustion air conduit 132. Further, as shown in FIG. 2, and in preferred embodiments of the invention, the combustion air inlet at the bottom 126 of the top feed tube 124 and the bottom combustion air inlet 134 not only lie along the same axis, such axis lies substantially normal to, and through, the center of the grate 114.

As shown, and in preferred embodiments, the cross section inside diameters of the top combustion air feed tube 148 and the bottom combustion air conduit inlet 134 are appreciably less than the cross-section inside dimensions of the combustion chamber 122. As depicted in FIG. 2, the cross-section inside diameters of the top combustion air feed tube 148 and the bottom combustion air conduit inlet 134 are about the same, and both are from about one-fourth to about one-third of the dimensions of the combustion chamber 122.

The top combustion air feed tube 148 as shown in FIG. 2 and in preferred embodiments is substantially centered within the chamber housing 112 and substantially centered within the combustion chamber 122. Flue gas rises in the combustion chamber 122 in the space between the outer surface of the top combustion air feed tube 148 and the inner surfaces of the chamber housing 112. At about its upper end, the chamber housing 112 is open through a restriction member (not shown) to a heat exchanger (not shown) and then to an exhaust conduit or stovepipe 140 that runs to and through exit point high on the perimeter wall 146. An exhaust fan 150 is positioned at the external distal end of the stovepipe 140, which draws flue gases and any entrained particulates out of the system.

The top combustion air feed tube 148 as shown in FIG. 2 and in some embodiments has a horizontal cross-section area of about 1.5 square inches, which takes up about one-twentieth of the approximately 25 square inches of the cross-sectional area of the chamber housing 112. This leaves about a cross-sectional area of about 23.5 square inches or so for the passage of flue gases up the combustion chamber 122, through the orifice in the restriction member (not shown) into, and then through, first a heat exchanger (not shown) and then the stovepipe 140.

The fuel pellets 123 are combusted on and/or slightly above the grate 114. Combustion air inflow from below the grate 114, together with the rising flow of flue gases, are routinely sufficient to lift or float pellets, particularly partially consumed pellets, slightly off and above the grate 114. Such a fire is being fed oxygen-containing combustion air from both above and below, and the flue gases emanating from the fire are discharged from the combustion chamber 122. During the operation of the pellet stove 110, each individual pellet of the single-layer of fuel pellets will burn to the point of almost complete combustion. In addition, the flue gases themselves will combust as they emanate from the ignited pellets. As fresh combustion air strikes the burning pellets 113 from above, it restokes the combustion bed every few seconds, thereby causing continuous flux within the combustion chamber 122, but not to the degree seen in other embodiments in which the pellets are accelerated downward with the combustion air. The pellet stove 110 of the present invention, because of the vertical sides and flat bottom the of burn pot 116, will probably, as mentioned above, require an ash collection device beneath the grate 114, such as the ash collection member 121 shown, to contain sediment from embers that fall through the grate 114 as mentioned above. The selection of a suitable ash collection device is well within the skill of a person with ordinary skill in the art. The provision of such an ash collection device beneath the grate 114 or in some other embodiments does not, however, take such embodiment outside of the scope of at least the broader embodiments of the present invention.

The grate 114 is an open framework or grid of bars or wires having a structural integrity that is sufficient to support the pellets on its upper side. As shown, and in preferred embodiments of the present invention, the grate 114 is level, i.e., horizontal with a substantially even flat top surface. By substantially even and flat is meant that there are substantially no hollows or other concave depressions of sufficient dimensions wherein pellets could lodge and build up atop of one another. The structural elements of any open framework, such as the grate 114, may themselves present minor ridges and/or small hollows between the elements provided that they are sufficiently small and/or shallow to substantially avoid pellet build-up.

The flat horizontal design of the grate 114 has been surprisingly found to provide optimum burn characteristics. It allows the pellets to combust individually, side by side, rather than in a heap such as the pellet-atop-pellet heaps required to maintain combustion in conventional pellet stove. The side-by-side combustion of pellets of the present invention achieves substantially complete pellet combustion without formation of fused ash. Fused ash is discussed above. In the substantial absence of fused-ash formation, the grate 114 does not become coated or clogged with fused ash, and the grate 114 does not require frequent and/or repetitive cleanings.

In addition, the grate 114 has sufficient gaps between its structural elements across its entire effective pellet-support surface to accommodate combustion air flow from the bottom combustion air inlet 134. In other words, combustion air preferably should permeate the grate 114 from below substantially evenly across the entire pellet-support surface of the grate 114.

Unlike current forced-draft blower systems, the exhaust fan 150 is disposed at the distal (and building-exterior) end of the stovepipe 140. The building-external exhaust fan 150, particularly in combination with the present system having combustion air inlets both below and above the burn grate 114, increases the air transfer, augmenting the combustion process in the burn chamber 122, but without the significant drawbacks of current forced-draft blower systems. One drawback avoided is the sound of the exhaust fans. Unlike current draft blower systems, the pellet stove 110 has no building-interior fan. Building-interior fans are typically sufficiently noisy to be an irritant in residential and workplace settings. The sound of the building-exterior exhaust fan 150 would not be heard within at all, or at most be heard as a soft murmur from inside.

Unlike current pellet stoves, the combustion in the combustion chamber 122 is so complete that the building-exterior exhaust fan 150 does not become clogged with deposits from the flue gases which pass through the fan 150. A building-exterior exhaust fan, such as exhaust fan 150, is generally impractical for conventional pellet stoves because it would be mounted outside high on a perimeter wall or on the roof, rendering intermittent frequent cleanings an irksome or overwhelming task. An exhaust fan used with various prototypes of the pellet stove of the present invention has been in operation heating a residential home for more than eighteen months, or three winters, without any need for cleaning.

Another drawback of current forced-draft blowers avoided by the building-exterior exhaust fan 150 is the risk of contamination of the interior environment presented by pushing flue gases into a stovepipe. The stovepipe 140 runs from the heat exchanger into which the combustion chamber 122 exhausts, to the exit point, which is the place or spot at which it penetrates the ceiling or, as shown, a point high on a wall 146, and from there exits to the outside. A significant length of the stovepipe 140 therefore is exposed within the room. Pushing flue gas into a stovepipe not only scours the stovepipe's internal surfaces, but it can also force flue gases through tiny gaps at the pipe seams and through cracks or tiny holes along the length of the pipe. Flue gases routinely contain carbon monoxide and other pollutants, such as wood ash. Even a minor escape of flue gas from a stove pipe into a building's interior environment poses a dangerous health hazard. As shown in FIG. 2, there are no forced-draft blowers downstream of the point at which the stovepipe 140 exits the room through the perimeter wall 146. Flue gases are sucked into the stovepipe 140 by the external exhaust fan 150 and there will be far less, if any, forcing of flue gases through tiny gaps at the pipe seams (not shown) and through cracks or tiny holes along the length of the stovepipe 140.

Further, the stovepipe-end exhaust fan 150 reduces cavitation within the intake-air/exhaust flow. Cavitation is the rapid formation and collapse of pressure cavities within an exhaust stream. When a blower is used to push exhaust gases out through a stovepipe, cavitation will occur. Cavitation results in a fluctuation and oscillation of the exhaust air stream within a stove, which in turn leads to a fluttering sound emitted by the stove and a reduction in combustion efficiency. A conventional spacious combustion chamber typically has ample room to absorb cavitation, but that same room creates combustion inefficiencies. Sucking the exhaust out of a stovepipe, such as by the use of the stovepipe-end exhaust fan 150, substantially eliminates cavitation, thereby providing an even flow of intake air and exhaust gasses, without flow fluctuations. Further, the resultant smooth combustion airflow further improves combustion efficiencies.

The stovepipe-end exhaust fan of the present invention, such as exhaust fan 150 illustrated in FIG. 2, is for the reasons described above an externally mounted draft inducer that is part of the engineered system of the biomass pellet combustion device of the present invention.

Referring now to FIG. 3 there is shown a pellet fuel heating device designated generally by the reference numeral 210. The pellet stove 210 as shown includes a burn pot 211 and an upright, tubular and transparent chamber housing 212 having a substantially circular cross-sectional profile. The burn pot 211 includes a grate 214 and a pot member 216. As shown, the pot member 216 is an open-topped receptacle having a substantially circular cross-sectional profile. The pot member 216 has a rim 218 or edge running along the entire upper perimeter of the pot member 216. The inner edge of the rim 218 forms the perimeter of an upwardly-open mouth 220 of the pot member 216.

The upright chamber housing 212 is shown in FIG. 3 in its operational placement, namely mated with the pot member 216. In the embodiment shown in FIG. 3, the chamber housing 212 is seated on the rim 218 of the pot member 216, and secured thereto by conventional mechanical fasteners such as the screw jack system 217 shown raising the pot member 216 sufficiently so that its rim 218 bears against the lower edge of the chamber housing 212, whereby the mating of the chamber housing 212 and the pot member 216 is realized.

The grate 214 as shown in FIG. 3 is seated on, and supported by, a support grid 219 affixed to the pot member 216. The grate 214 has a substantially circular cross-section profile, has a low upright perimeter enclose or wall 215, and is spaced apart from the chamber housing 212 at all points along its perimeter. The upwardly-open mouth 220 of the pot member 216 and the grate 214 both have a diameter of about five inches. The upright perimeter wall 215 of the grate 214 is somewhat less than one inch high. The chamber housing 212 and pot member 216, in combination, form, determine and/or enclose a burn or combustion chamber 222.

As shown, and in preferred embodiment, the combustion chamber 222 is a close-mode or constricted burn chamber. The combustion chamber 222 has a diameter of about five inches and is about thirty-eight inches long (high). Although the diameter of the grate 214 is about an inch less (about 20 percent less) than the inside diameter of the pot member 216, the combustion chamber 222 still is a confined or close-mode combustion chamber which permits greater thermal efficiencies than the efficiencies possible with current pellet stoves. The close-mode combustion chamber 222 also provides a significant particulate emissions reduction over emissions given out by current pellet stoves.

The pellet stove 210 shown in FIG. 3 includes a pellet feed tube 224. The pellet feed tube 224 is a conduit through which pellets 223 are routed from a pellet hopper (not shown) directed onto the grate 214. The pellets 223 are conveyed from the hopper (not shown) into the pellet feed tube 224 by a conventional auger screw device or the like (not shown). The pellets 223 are pushed or directed onto the grate 214 from the side.

The pellet stove 210 shown in FIG. 3 includes a top combustion air conduit 230 that forks off of a main combustion air conduit 228 and runs over to a point above the combustion chamber 222, through the otherwise closed chamber housing 212, and then down through the combustion chamber 222, terminating at a point substantially centered above the grate 214. The portion of the top combustion air conduit that is disposed inside of the chamber housing 212 when the pellet stove 210 is in operational assembly can be considered a top combustion air feed tube 248. It also includes a stove pipe 240 open to the burn chamber 227.

The embodiment shown in FIG. 3 as described so far differs from the embodiments shown in FIG. 1 and FIG. 2 in several respects, including without limitation (1) the transparency of the combustion chamber housing 212, (2) the perimeter wall 215 of the grate 214, (3) the gap between the grate 214 and the inner surface of the chamber housing 212, and (4) a pellet tube 224 that deposits the pellets 213 at the side of the grate 214 rather than dropping the pellets 213 from an elevated position. These variations are not believed as advantageous as the features shown in, and described with respect to, FIG. 1, but nonetheless are well within broad embodiments of the invention. Individually or in combination these variations may find different uses and benefits not delineated herein and still provide at least some of the advantages of the present invention and thus remain within the present invention.

The top combustion feed tube 248 has a distal bottom open end 226. The open end 226 of the top air feed tube 248 is sufficiently close to the grate 214 to provide sufficient turbulence in the vicinity of the grate 214. The pellets 223 to some extent actually bounce around the grate 214 under the influence of the air turbulence, whereby a substantially single layer of pellets 213 under constant agitation is maintained. A pile or other accumulation or buildup of pellets on the grate 214 is neither required nor desirable for the pellet stove 210 of the present invention, and is avoided despite the entry of the pellets 223 from the side in the tube 224.

As shown, and in some embodiments of the invention, the open end 226 of the top combustion air feed tube 248 is spaced above the five-inch diameter grate 214 a distance of about eight inches when the pellet stove 210 is in operational assembly as shown in FIG. 3. In such an embodiment, a burn chamber center 227 bounded by the grid 219 which supports the grate 214, the chamber housing 212, and the horizontal plane of the open end 226 of the top combustion air feed tube 248 is demarcated. It is noted here that the upper combustion air inlet should be sufficiently close to the pellet-supporting surface of the grate to provide the required turbulence, but not so close as to blow pellets to the side of the grate or the pot element.

The pellet stove 210 shown in FIG. 3 includes a second or bottom combustion air conduit 232 which runs from a source of combustion air (not shown) over to a point below the burn pot 211, then up through the otherwise closed bottom of the pot member 216. The bottom combustion air conduit 232 opens into the bottom of the pot member 216 beneath the grate 214 at a bottom combustion air inlet. The bottom combustion air inlet is substantially centered on the bottom of the pot member 216. As shown in FIG. 3, and in some embodiments of the invention, the pot member 216 is about 3 inches high. Therefore the grate 214 of the pellet stove 210 shown in FIG. 3, when seated on the grid 219 in the pot member 216, will be positioned between the two combustion air inlets, namely the top combustion air inlet at the open end 226 of the top combustion air feed tube 248 and the bottom combustion air inlet 234 at the end of the bottom combustion air conduit 232. Further, as shown in FIG. 3, and in preferred embodiments of the invention, the top combustion air inlet at the bottom 226 of the top feed tube 248 and the bottom combustion air inlet 234 not only lie along the same axis, such axis lies substantially normal to, and through, the center of the grate 214.

As shown, and in preferred embodiments, the cross section inside diameters of the top combustion air feed tube 248 and the bottom combustion air conduit inlet are appreciably less than the cross-section inside diameters of the combustion chamber 222. As depicted in FIG. 3, the cross-section inside diameters of the top combustion air feed tube 224 and the bottom combustion air conduit inlet are about the same, and both are from about one-fifth to about one-fourth of the diameters of the combustion chamber 222.

The top combustion air feed tube 248 as shown in FIG. 3 and in preferred embodiments is substantially centered within the chamber housing 212 and substantially centered within the combustion chamber 222. Flue gas rises in the combustion chamber 222 in the space between the outer surface of the top combustion air feed tube 248 and the inner surfaces of the chamber housing 212. At about its upper end (not shown), the chamber housing 212 is open to an exhaust conduit or stovepipe (not shown) that runs to and through exit point high on a perimeter wall or ceiling (not shown). An exhaust fan (not shown) is positioned at the external distal end of the stovepipe, which draws flue gases and any entrained particulates out of the system, as is described above regarding the embodiment shown in FIG. 2.

The fuel pellets 213 are combusted on and/or slightly above the grate 214. Combustion air inflow from below the grate 214, together with the rising flow of flue gases, are routinely sufficient to lift or float pellets, particularly partially consumed pellets, slightly off and above the grate 214. Such a fire is being fed oxygen-containing combustion air from both above and below, and the flue gases emanating from the fire are discharged from the combustion chamber 222 through the heat exchanger (not shown) and then to the stovepipe (not shown). During the operation of the pellet stove 210, each individual pellet of the single-layer of fuel pellets will burn to the point of almost complete combustion. In addition, the flue gases themselves will combust as they emanate from the ignited pellets. Superior combustion, and superior thermal energy, is produced by the downward flow of combustion air through the top combustion air feed tube 248. As fresh combustion air strikes the burning pellets 213 from above, it restokes the combustion bed continuously, thereby causing continuous flux within the combustion chamber 222. Such a flux or dynamic mode results in complete, or near complete, burning of the pellets 213 and their embers. Such combustion efficiencies vastly increase thermal output and dramatically reduce the fly ash and particulate content of the flue gas. The pellet stove 210 of the present invention, if operated with at least a modicum of proficiency, requires at most a simple ash collection device 221 having an ash collection housing 221 a and a normally-closed drawer 221 b (shown open for cleaning) as shown beneath the grate 214 to contain accumulating ash as the fire burns, simply because little to no ash accumulates. The provision of such an ash collection device 221 beneath the grate 214 with a collection drawer for unburned particulates is generally desirable to avoid plugging of the bottom air inlet.

Both the grid 219 and the grate 214 are open frameworks or grids of bars or wires having a structural integrity that is sufficient to support respectively the grate and the pellets on their upper sides. As shown, and in preferred embodiments of the present invention, the pellet supporting surface of grate 214 is level, i.e. flat with a substantially even flat top surface. By flat is meant that there are substantially no hollows or other concave depressions of sufficient dimensions wherein pellets could build up atop of one another. The structural elements of any open framework, such as the grate 214, may themselves present minor ridges and/or small hollows between the elements provided that they are sufficiently small to substantially avoid pellet build-up.

The flat horizontal design of the pellet-supporting surface of the grate 214 has been surprisingly found to provide optimum burn characteristics. It allows the pellets to combust individually, side by side, rather than in a heap such as the pellet-atop-pellet heaps required to maintain combustion in conventional pellet stoves. The side-by-side combustion of pellets of the present invention achieves substantially complete pellet combustion without formation of fused ash. Fused ash is discussed above. In the substantial absence of fused-ash formation, the grate 214 does not become coated or clogged with fused ash, and the grate 214 does not require frequent and/or repetitive cleanings.

In addition, the grate 214 and the underlying grid 219 have sufficient gaps between their structural elements to accommodate combustion air flow from the bottom combustion air inlet. In other words, combustion air preferably should permeate the grate 214 from below substantially evenly across the entire pellet-support surface of the grate 214.

Referring now to FIG. 4 there is shown a portion of a reverse-direction combustion-chamber pellet fuel heating device, or pellet stove, designated generally by the reference numeral 310. The pellet stove 310 portion as shown includes an upright, tubular chamber housing 312 having a substantially circular cross-sectional profile. Within the upper regions of the chamber housing 312 is a pellet supporting grate 314. Above the grate 314 is the hemispherical-narrowing to a combined air/fuel inlet 332. The pellets 323 are shown dropping through the inlet 332 onto the grate 314. Even if a reverse-direction embodiment, such as that shown in FIG. 4, is considered an upside-down alternative, the pellets 323 must be loaded onto the actually-up side of the grate 314.

A concentric combustion air conduit 330 supplies a combustion air flow below the grate 314 with its outlet facing the grate. A top combustion air conduit 331 feeds combustion air to the combined air/fuel inlet 332. As shown by the air-flow arrows on FIG. 4, the combustion chamber exhausts at the bottom, not the top, through stove pipe 340. In such a reverse-direction combustion chamber pellet stove, the flaming off the burning pellets would be downwards, in the direction of the exhaust outlet from the combustion chamber. In this reverse-direction combustion-chamber pellet fuel heating device, with a grate 314 positioned at the top of the combustion chamber, rather than at the bottom, an ash collection member 321 is conveniently formed as a bottom end section as shown.

Also exemplified by the embodiment shown in FIG. 4 is a construction of the chamber housing and burn pot as a single integrated unit. Such unitary construction is not limited to a reverse-direction combustion chamber, and the pellet stoves illustrated in FIG. 1-3 could likewise have been fabricated with a chamber housing/burn pot unit combination.

The burning of pellets in a pellet heating device of the present invention can be started in any conventional manner, such as through high-resistance electrical wires in contact with, or in close association with, the pellets on the grate.

In preferred embodiments of the invention, the size of the close-mode combustion chamber, that is volumetric size of the space between the end of the upper combustion air inlet and the burn grate, is selected by, or matched to, the volume of air moved per unit time by the combustion air (exhaust) fan. Of significance, however, is the fact that the pellet stove of the present invention runs with a greater air to fuel ratio than conventional pellet stoves. In other words, more combustion air flows through the pellet stove for a given amount of pellet fuel than conventional pellet stoves.

In preferred embodiments of the invention, the biomass pellet fuel is exposed to fresh combustion air from above and below in a confined or close-mode combustion chamber. In preferred embodiments of the invention, pellet combustion device has a downwardly facing combustion air inlet positioned from about five to about twenty inches above the pellet grate, and more preferably from about eight to about fifteen inches above the pellet grate. In preferred embodiments of the invention, pellet combustion device has an upwardly facing combustion air inlet positioned from about two to about ten inches below the pellet grate, and more preferably from about three to about six inches below the pellet grate.

In preferred embodiments of the invention, the biomass pellet fuel is exposed to sufficient combustion air flow and the fuel pellets are sufficiently spread or strewn-out over the grate that the fuel pellets are not static, and instead are being stoked by the impingement of air flow, substantially without any pellet heaping. In further preferred embodiments, the fuel pellets on the grate are being stoked by fuel pellets dropping from a pellet feed device down to the grate.

In preferred embodiments of the invention, the confined or close-mode combustion chamber has a cross-sectional profile within the range of from about ten square inches to about thirty-five square inches, and more preferably from about twelve square inches to about thirty square inches. In preferred embodiments of the invention, the confined or close-mode combustion chamber has a burn center, namely the space within the burn chamber housing from the top of the grate to the upper combustion air inlet, has a volumetric size of from about fifty to four hundred cubic inches, and more preferably from about one hundred to about three hundred cubic inches.

In preferred embodiments of the invention, the confined or close-mode combustion chamber no more than about ten inches wide, and more preferably no more than about six or seven inches wide. In preferred embodiments of the invention, the grate substantially level or flat, and extends up to the sides of the burn chamber housing, or at least no more than about an inch away from the burn chamber housing.

The present invention provides, in some embodiments, a pelletized-biomass combustion device comprising: (a) an elongate upright combustion capsule; (b) a grate element disposed within and bisecting the combustion capsule, (i) the grate element having a first side and a second side and (ii) the grate element open to fluid communication between the first side and the second side; (c) a first combustion air inlet facing the first side of the grate; (d) a second combustion air inlet facing the second side of the grate; (e) a fuel-feed inlet; and (f) a flue-gas exhaust outlet. For example, referring to FIG. 3 where there is shown a portion of pellet fuel heating device or pellet stove 210 which includes an upright, tubular chamber housing 212 and a burn pot 211 having a grate 214 and a pot member 216, the chamber housing 212 and the pot member 216 in combination comprise such an elongate upright combustion chamber bisected by the grate 214. A combustion capsule of the present invention can be formed of two mated components such as the chamber housing 212 and pot member 216, or instead it can be formed as a single unit such as the chamber housing 310 bisected by the grate 314 illustrated in FIG. 4. In any event, a grate element such as grate 214 or grate 314 has two sides, and there are two combustion air inlets, each facing opposite sides of the grate, such as the open end 226 of the top combustion air feed tube 248 and the open end of the bottom combustion air conduit 232 shown in FIG. 3. It is noted that while the grate elements, such as 214 shown in FIG. 3, are illustrated as having parallel flat sides, the second side of any grate element can have any of a virtually unlimited number of configurations without departing from the scope of the present invention, provided there is sufficient fluid communication between the two sides of a grate element for the circulation of the combustion air and flue gas as described herein.

It is further noted that the combustion air inlets of the present invention are substantially facing a side of the grate element, whereby combustion air is directed at a side of the grate element, and this condition is not met, for instance, by merely an air inlet below the grate directed parallel to the grate. This condition is typically best met with an air inlet directing the combustion air in a stream that runs normal to the side of the grate, and the stream should at least run to, and impinge or impact the side of the grate. It is also noted that the grate element can be supported on a ridge or other protrusion from the side of the pot member, or even the chamber housing, and still bisect the combustion capsule. In other words, the bisecting is a division of the capsule into upper and lower chambers, or as mentioned below, primary and secondary combustion chambers.

Continuing, the grate element has a substantially horizontal and substantially flat fuel-support surface open to the fuel-feed inlet. The fuel-support surface can be essentially the entire upper surface of the grate, such as shown for grate 14 illustrated in FIG. 1, or instead the region within the perimeter wall 215 of the grate 214 illustrated in FIG. 3. The fuel-support surface, or pellet-supporting surface, is that portion of the grate element to which fresh pellets are delivered, and on which the pellet bulks are combusted. This surface in preferred embodiments is sufficiently horizontal and sufficiently flat so that the pellets are, and remain, distributed thereon in a substantially single, and possibly discontinuous, layer, rather than heaping up. As mentioned above, heaps of pellets on the supporting structure is neither required nor desirable for the present invention. In any event, the fuel-support surface is normally open to the fuel-feed inlet. In other words, such surface and the feed inlet must be positioned so that pellets exiting the feed inlet are received at the fuel-support surface.

Continuing, in preferred embodiments of the invention the fuel-support surface substantially extends across the combustion capsule providing a close-mode cross-section profile to the combustion capsule. In other words, the fuel-support surface, substantially along its entire perimeter, reaches to, or almost to, the internal sides of the combustion capsule, such as shown for grate 14 illustrated in FIG. 1, or instead the region within the perimeter wall 215 of the grate 214 illustrated in FIG. 3. Unlike conventional pellet stoves, the present invention's restricted-mode or close-mode of combustion permits and provides high external surface temperatures (particularly when the chamber wall is of thin-wall construction), low internal negative pressures, high combustion efficiencies and other advantages noted herein. Further it is believed that a close-mode combustion will not provide the various advantages, or at least not provide all of the various advantages, unless paired with dual combustion-air streams directed at the combusting fuel pellets from above and below. The fuel-support surface should extend sufficiently close to the chamber walls to provide a close-mode combustion, and it is believed that fuel-support surface having, for instance, a diameter of at least about 70 percent of the cross-sectional inside diameter of chamber housing will provide a close-mode combustion, at least to a significant degree, while a fuel-support surface having a diameter of at least about 80 or 90 percent of the cross-sectional inside diameter of chamber housing will provide a close-mode combustion to a significant degree.

Continuing, in preferred embodiments of the invention the pelletized-biomass combustion device includes a stovepipe downstream of the flue-gas exhaust outlet with an exhaust fan in fluid communication with the stovepipe and disposed proximate the distal end of the stovepipe, which for example is illustrated by the exhaust fan 150 positioned at the external distal end of the stovepipe 140 shown in FIG. 2. It is believed that such a remote, building-external exhaust fan is not a viable or practical possibility for pellet stoves outside of the present invention because such a fan would become fouled far too frequently in the absence of combustion efficiencies provided by the present invention's close mode combustion in the presence of dual air streams directed at the combusting fuel from both below and above. In addition, as seen from all the embodiments illustrated and described in detail herein, not only is such a remote, building-external exhaust fan possible in the present invention, the various embodiments of the invention illustrated require no other mechanism for circulating combustion air through the combustion capsule and exhausting flue gases out of the combustion capsule. Although the inclusion of a fan pushing combustion air into the combustion capsule would typically not be a departure from the present invention that would so defeat its fundamental constructs and advantages so as to be outside of its scope, there generally would be no advantage to such a fan justifying its use in addition to, or in substitute for, a remote, building-external exhaust fan. Such a remote exhaust fan should move sufficient volumes of flue gas given the size of the combustion capsule and the amount of fuel being combusted herein per unit time. The various embodiments explicitly illustrated herein are sized for home pellet-stove use, and large commercial versions would require a scale-up in the size of the exhaust fan, which is well within the skill of a person with ordinary skill in the field.

Continuing, in some of the preferred embodiments of the invention the grate element bisects the combustion capsule into an upper chamber and a lower chamber, for example as illustrated by the grate 14 of the pellet stove 10 shown in FIG. 1. The first side of the grate element is open to the upper chamber, and the second side of the grate element is open to the lower chamber. As illustrated in FIG. 1, the combustion chamber 22 and any internal portion of the pot member 16 faced by the up-side first side of the grate 14 comprises the upper chamber. The internal region of the pot member 16 faced by the down-side second side of the grate 14 comprises the lower chamber. In the embodiments illustrated, except the upside-down embodiment shown in FIG. 4, the upper chamber can also be considered a primary combustion chamber, and the lower chamber can also be considered a secondary combustion chamber. In the embodiment shown in FIG. 4, the reverse designations are appropriate.

Continuing, in preferred embodiments of the invention the lower or secondary chamber has a substantially hemispherical configuration, such as that illustrated for the internal region of the pot member 16 below the grate 14 of the pellet stove 10 of FIG. 1. By substantially hemispherical is meant at least sides sloping and converging down to the bottom, for example a cone configuration at an incline of at least about 35 or 45 degrees from vertical. Although there is believed no particular advantage to any departure from a hemispherical configuration, a precisely hemispherical configuration is not required to provide at least some of the mechanisms and/or advantages of the present invention.

Continuing, in preferred embodiments of the invention the upper or primary combustion chamber is substantially bounded by a sufficiently thin-wall-construction chamber housing. As mentioned above, the wall of a chamber housing of a pelletized-biomass combustion device of the present invention, sized for in-room residential heating, can be as thin as 1/16 inch, and typically will be of sufficiently thin-wall-construction if it is no more than ¼ of an inch thick. Large commercial versions would require a scale-up in the size of the upper or primary combustion chamber, and the appropriate thickness for a sufficiently thin-wall-construction chamber housing might be two inches or more. Such a scaling up is well within the skill of a person with ordinary skill in the field.

Continuing, in preferred embodiments of the invention the combustion air/fuel feed tube are coextensive, and the first combustion air inlet, the fuel-feed inlet and the open end of the coextensive combustion air/fuel feed tube are coincident, as illustrated in the pellet stove 10 of FIG. 1.

Continuing, in preferred embodiments the present invention includes a method of combusting biomass fuel pellets in a pelletized-biomass combustion device under flux conditions within a confined combustion capsule. The pelletized-biomass combustion device used in the method is itself an embodiment of the invention, namely one having an elongate upright combustion capsule, a grate element (a) disposed within and bisecting the combustion capsule, (b) having a first side and a second side, (c) open to fluid communication between the first side and the second side, (d) having a substantially horizontal and substantially level fuel-support surface open to the fuel-feed inlet and (e) substantially extending across the combustion capsule providing a close-mode cross-section profile to the combustion capsule, as illustrated in FIG. 1 among other examples herein. Also as illustrated and described herein, the combustion device has a first combustion air inlet facing the first side of the grate, a second combustion air inlet facing the second side of the grate, a fuel-feed inlet and a flue-gas exhaust outlet. The method comprises the steps of: combusting pelletized-biomass fuel on the fuel-support surface while feeding combustion air to the fuel-support surface simultaneously through the first combustion air inlet and the second combustion air inlet. In further preferred embodiment, wherein the combustion device further includes a stovepipe downstream of the flue-gas exhaust outlet and an exhaust fan in fluid communication with the stovepipe and disposed proximate the distal end of the stovepipe, the method further includes the step of maintaining a water-column negative draft pressure within the combustion capsule of between about 0.01 to about 0.08 while combusting the pelletized-biomass fuel on the fuel-support surface. Such a range of water-column negative draft pressures were actually measured in a pellet stove of the present invention using a digital manometer that measured the internal pressure in terms of inches of water column. Such a low internal-pressure range is not achievable in conventional pellet stoves.

Continuing, in preferred embodiments the method, wherein the grate bisects the combustion capsule into a primary combustion chamber and a secondary combustion chamber as described above, and wherein the primary combustion chamber is bounded by a thin-wall-construction chamber housing having an external surface, the method further includes the step of maintaining a temperature of about 400° F. to about 2,000° F. at the external surface of the chamber housing while combusting the pelletized-biomass fuel on the fuel-support surface. In more detail, in addition to other comments above, the combination of a close-mode combustion chamber and a thin-wall-construction chamber wall provides far higher external-surface temperatures than those seen in conventional pellet stoves. Further, at a pilot setting, the external surface temperature of the combustion chamber housing can be as low as from about 400° F. to about 600° F. At a high burn rate, the external surface temperature of the combustion chamber housing can reach temperatures from about 1,000° F. to about 1,600° F. Such high external temperatures are not even approached in a conventional pellet stove.

Continuing, in preferred embodiments the method, wherein the fuel-feed inlet is above and spaced-apart from the fuel-support surface, the method further includes the step of feeding fresh pelletized-biomass fuel to the pelletized-biomass fuel on the fuel-support surface by dropping the fresh pelletized-biomass fuel from the fuel-feed inlet onto the fuel-support surface while combusting the pelletized-biomass fuel on the fuel-support surface. In such embodiment, the impingement of the fresh pellets augments the flux conditions, and further prevents a continuous line of fresh pellets that would be a backburn risk as described above.

While the foregoing written description of the invention enables one of ordinary skill in the art to make and use the invention, and to make and use what is presently considered the best mode of the invention, those of ordinary skill in the art will understand and appreciate the existence of variations, combinations and equivalents of the specific embodiments, methods and examples provided herein. The present invention should not be limited by the above described embodiments, methods and examples. 

1. A pelletized-biomass combustion device comprising: an elongate upright combustion capsule; a grate element disposed within and bisecting said combustion capsule, said grate element having a first side and a second side; said grate element open to fluid communication between said first side and said second side; a first combustion air inlet facing said first side of said grate; a second combustion air inlet facing said second side of said grate; a fuel-feed inlet; and a flue-gas exhaust outlet.
 2. A pelletized-biomass combustion device according to claim 1 wherein said grate element has a fuel-support surface open to said fuel-feed inlet, and wherein said fuel-support surface is substantially horizontal and substantially flat.
 3. A pelletized-biomass combustion device according to claim 1 wherein said grate element has a fuel-support surface open to said fuel-feed inlet, wherein said fuel-support surface is substantially horizontal and substantially level, wherein said fuel-support surface substantially extends across said combustion capsule providing a close-mode cross-section profile to said combustion capsule.
 4. A pelletized-biomass combustion device according to claim 1 further including a stovepipe downstream of said flue-gas exhaust outlet, said stovepipe having a proximal end and a distal end; and an exhaust fan in fluid communication with said stovepipe and disposed proximate said distal end of said stovepipe.
 5. A pelletized-biomass combustion device according to claim 1 wherein said grate element bisects said combustion capsule into an upper chamber and a lower chamber, wherein said first side of said grate element is open to said upper chamber, wherein said second side of said grate element is open to said lower chamber, wherein said first side of said grate element is substantially horizontal and level, and is open to said fuel-feed inlet, wherein said first combustion air inlet is disposed within said upper chamber and is spaced apart from, and substantially centered on, said first side of said grate element, wherein said second combustion air inlet is disposed within said lower chamber and is spaced apart from, and substantially centered on, said second side of said grate element, and wherein said flue-gas exhaust outlet is open to said upper chamber.
 6. A pelletized-biomass combustion device according to claim 5 wherein said first side of said grate element includes a fuel-support surface open to said fuel-feed inlet, wherein said fuel-support surface is substantially horizontal and level, and wherein said fuel-support surface substantially extends across said combustion capsule providing a close-mode cross-section profile to said combustion capsule.
 7. A pelletized-biomass combustion device according to claim 6 wherein said lower chamber has a substantially hemispherical configuration.
 8. A pelletized-biomass combustion device according to claim 6 wherein said upper chamber is substantially bounded by a thin-wall-construction chamber housing.
 9. A pelletized-biomass combustion device according to claim 6 further including a coextensive combustion air/fuel feed tube disposed within said upper chamber, said coextensive combustion air/fuel feed tube having an open end, wherein said first combustion air inlet, said fuel-feed inlet and said open end of said coextensive combustion air/fuel feed tube are coincident.
 10. A pelletized-biomass combustion device comprising: an elongate upright combustion capsule comprised of a combustion chamber housing and a burn pot mated with said combustion chamber housing; a grate element disposed within said combustion chamber, said grate element and said combustion chamber housing substantially defining a primary combustion chamber, said grate element and said burn pot substantially defining a secondary combustion chamber; said grate element having a first side facing said primary combustion chamber; said grate element having a second side facing said secondary combustion chamber; said primary combustion chamber being in fluid communication with said secondary combustion chamber through said grate element; a first combustion air inlet facing said first side of said grate; a second combustion air inlet facing said second side of said grate; a fuel-feed inlet; and a flue-gas exhaust outlet from said primary combustion chamber.
 11. A pelletized-biomass combustion device according to claim 10 wherein said secondary combustion chamber has a substantially hemispherical configuration.
 12. A pelletized-biomass combustion device according to claim 10 wherein said secondary combustion chamber has a bottom, and wherein said second air inlet is disposed at said bottom of said secondary combustion chamber.
 13. A pelletized-biomass combustion device according to claim 10 wherein said first side of said grate element has a fuel-support surface open to said fuel-feed inlet, and wherein said fuel-support surface is substantially horizontal and substantially flat.
 14. A pelletized-biomass combustion device according to claim 10 wherein said grate element has a fuel-support surface open to said fuel-feed inlet, wherein said fuel-support surface is substantially horizontal and substantially level, wherein said primary combustion chamber has a horizontal cross-section profile, and wherein said fuel-support surface is substantially coincident with said horizontal cross-section profile of said primary combustion chamber.
 15. A pelletized-biomass combustion device according to claim 10 further including a stovepipe downstream of said flue-gas exhaust outlet, said stovepipe having a proximal end and a distal end; and an exhaust fan in fluid communication with said stovepipe and disposed proximate said distal end of said stovepipe.
 16. A pelletized-biomass combustion device according to claim 10 further including a first air feed tube, said first air feed tube being substantially disposed within said primary combustion chamber and having a terminal end, and said first air inlet being coincident with said terminal end of said first air feed tube.
 17. A pelletized-biomass combustion device according to claim 10 wherein said combustion chamber housing is a substantially thin-wall-construction combustion chamber housing.
 18. A pelletized-biomass combustion device according to claim 10 wherein said first combustion air inlet is substantially centered on said first side of said grate, and wherein said second combustion air inlet is substantially centered on said second side of said grate.
 19. A method of combusting biomass fuel pellets in a pelletized-biomass combustion device under flux conditions within a confined combustion capsule, said pelletized-biomass combustion device having an elongate upright combustion capsule and a grate element disposed within and bisecting said combustion capsule, said grate element having a first side and a second side, and said grate element open to fluid communication between said first side and said second side, said grate element having a substantially horizontal and substantially level fuel-support surface open to said fuel-feed inlet and substantially extending across said combustion capsule providing a close-mode cross-section profile to said combustion capsule, said combustion device further having a first combustion air inlet facing said first side of said grate, a second combustion air inlet facing said second side of said grate, a fuel-feed inlet and a flue-gas exhaust outlet, comprising the steps of: combusting pelletized-biomass fuel on said fuel-support surface while feeding combustion air to said fuel-support surface simultaneously through said first combustion air inlet and said second combustion air inlet.
 20. A method of combusting biomass fuel pellets in a pelletized-biomass combustion device under flux conditions within a confined combustion capsule according to claim 19, wherein said combustion device further includes a stovepipe downstream of said flue-gas exhaust outlet, said stovepipe having a proximal end and a distal end, and an exhaust fan in fluid communication with said stovepipe and disposed proximate said distal end of said stovepipe, further including the step of: maintaining a water-column negative draft pressure within said combustion capsule of between about 0.01 to about 0.08 while combusting said pelletized-biomass fuel on said fuel-support surface.
 21. A method of combusting biomass fuel pellets in a pelletized-biomass combustion device under flux conditions within a confined combustion capsule according to claim 19, wherein said grate bisects said combustion capsule into a primary combustion chamber and a secondary combustion chamber, said first said of said grate element facing said primary combustion chamber, and said second side of said grate element facing said secondary combustion chamber, and said primary combustion chamber being bounded by a thin-wall-construction chamber housing having an external surface, further including the step of: maintaining a temperature of about 400° F. to about 2,000° F. at said external surface of said chamber housing while combusting said pelletized-biomass fuel on said fuel-support surface.
 22. A method of combusting biomass fuel pellets in a pelletized-biomass combustion device under flux conditions within a confined combustion capsule according to claim 19, wherein said fuel-feed inlet is above and spaced-apart from said fuel-support surface, further including the step of: feeding fresh pelletized-biomass fuel to said pelletized-biomass fuel on said fuel-support surface by dropping said fresh pelletized-biomass fuel from said fuel-feed inlet onto said fuel-support surface while combusting said pelletized-biomass fuel on said fuel-support surface. 